1. Field of the Invention
The present invention relates to glucan or derivatives thereof useful as raw materials in the starch processing industry, as a composition for drinks and foods, as a composition for food additives, as a composition for adhesion, or as starch substitutes for biodegradable plastic; and a method for producing the glucan or derivatives thereof.
More specifically, the present invention relates to glucan with a degree of polymerization of 50 or more having an inner branched cyclic structure portion and an outer branched structure portion and a method for producing the same.
2. Description of the Related Art
Starch is a polymer substance used as a material for producing maltose, starch syrups, or cyclodextrin, a composition for foods and drinks, a composition for food additives, and a material for adhesion or biodegradable plastic. However, existing starch has various problems. For example, it is likely to retrograde and it has high viscosity. In general, starch has low solubility in water, and starch is therefore required to be heat treated or treated with an organic solvent, acid, or alkali so as to be dissolved in water.
Dissolved starch or gelatinized starch rapidly retrogrades to form an insoluble precipitate. When starch is retrograded, its physical properties such as viscoelasticity, adhesion change, water holding ability, shape holding properties, freezing resistance, and/or digestiveness in foods containing the retrograded starch decrease.
Furthermore, gelatinized starch has high viscosity. This is due to the fact that amylopectin in starch is composed of long chain molecules having a number of branches. In the case where maltose or cyclodextrin is produced using starch as a raw material, gelatinized starch is difficult to handle because of its high viscosity. For example, when gelatinized starch having a concentration at a certain level is transported through a pipe, the pipe becomes clogged with starch.
As described above, the characteristics (low solubility, retrogradation, and high viscosity) of existing starch limit the use thereof in foods and other fields.
Under these circumstances, a study was conducted for improving the solubility and retrogradation resistance of starch by allowing the starch to decompose into smaller molecules. As a result, retrogradation of starch was suppressed to a certain degree. However, it is difficult to prevent an excess decrease in molecular weight, and intrinsic characteristics of starch, which is originally a macromolecule, are lost. Furthermore, according to these methods, the reducing power of starch increases. When such starch is heated while being mixed with protein and amino acid so that when starch reacts, it turns colors. This problem also limits the use of starch.
On the other hand, a study for improving the solubility of starch without decomposing it into smaller molecules was conducted. Branching enzyme (Q-enzyme, EC 2.4.1.18) cleaves an .alpha.-1,4-bond of starch and allows an .alpha.-1,6-bond to be synthesized with the resulting starch by its transglycosylation reaction. Branching enzyme was allowed to react with starch to obtain water-soluble starch (Japanese Laid-Open Patent Publication No. 60-75295). However, the water-soluble starch obtained by this method is a polymer substance having a molecular weight, as high as starch used as a raw material, and therefore, cannot solve the problems mentioned above.
As a substitute for the above-mentioned starch, cyclic sugars composed of D-glucose, that is, cyclic glucan has been considered.
Cyclodextrins (CD) are known cyclic gulcans. Cyclodextrins are produced by allowing cyclodextrin glucanotrasferase (hereafter, referred to as CGTase) to react with starch or the like. When CGTase is allowed to react with starch, cyclodextrins (.alpha.-CD, .beta.-CD, and .gamma.-CD) having a degree of polymerization of 6 to 8, respectively, are usually produced. .alpha.-CD, .beta.-CD, and .gamma.-CD cannot, however, be substitutes for starch; thus, cyclodextrin having a higher degree of polymerization is desired.
Cyclodextrins having a degree of polymerization of 9 to 13 are synthesized by allowing CGTase to react with starch (Archives of Biochemistry and Biophysics, vol. 111, pp. 153-165 (1965)); however, their yield is extremely low.
Although Japanese Laid-Open Patent Publication No. 6-62883 discloses CD having a high degree of polymerization, its degree of polymerization is as high as 28. Furthermore, Archives of Biochemistry and Biophysics, vol. 111, pp. 153-165 (1965) describes that glucan (inner branched glucan or inner branched CD) having an .alpha.-1,6-bond in a cyclic structure composed of an .alpha.-1,4-glucoside bond, or glucan (outer branched glucan or outer branched CD) having an .alpha.-1,6-bond outside of a cyclic .alpha.-1,4-glucan is produced as a by-product of the action of CGTase on starch.
Japanese Laid-Open Patent Publication No. 6-62883 describes the structure of inner branched glucan by illustrating cyclodextrin having an inner branched structure. This Publication describes that inner branched cyclodextrin is present only in large cyclic cyclodextrin having degree of polymerization of 10 to 13 in which large strain is applied; and that the number of .alpha.-1,6-bonds present in the ring was one in accordance with analysis.
Japanese Laid-Open Patent Publication Nos. 63-46201 and 64-74997 describe the structure of an outer branched glucan and the method for producing the same. These Publications describe that branched dextrin can be produced by reacting debranching enzyme with the mixture of cyclodextrin and linear or branched dextrin. However, since cyclodextrin used for the reaction has no inner branched cyclic structure, glucan having an inner branched cyclic structure portion and an outer branched structure portion are not disclosed. Furthermore, the cyclic structure portion has a degree of polymerization of 6 to 8, and the outer branched structure portion is merely bound by maltooligosaccharide or branched maltooligosaccharide having a degree of polymerization of as high as 6.
As described above, although cyclodextrin may be considered a substitute for the above-mentioned starch, the CDs above cannot be a substitute for starch because of their small degree of polymerization. Although inner branched or outer branched CDs are disclosed, those branched CDs also have small degree of polymerization, and they are products of trace yields, so that they cannot be used as a substitute for starch.
In view of the above-mentioned circumstances, a method for easily producing cyclic glucan having a large molecular weight capable of being a substitute for starch is desired.